If the pH of the final curd before boiling is below 6 it will cause texture issues and it may start to loose shape when heated. Acidity should be low and pH quite high. It is a forgiving cheese because there is no affinage. The cultures don't matter much since you really are just getting the milk flavor. Salting the curd before pressing will help keep the pH up. Salt levels should be 1.5 to 2% depending on your tastes.

Alex, thanks, dumb me, now I understand better, pH is high and acidity is low close to fresh milk as no starter culture.

Francoise, thanks, I was also thinking that adding starter culture wouldn't add much flavour before being boiled and killed, still I'll try that on next batch. I'll also salt the curd rather than the boiled cheese as you propose, 1.5-2% is of final weight.